Human A&PBIO 304 · American River College ← Week 04 pre-work

BIO 304 · Week 04 · Interactive Workbook

Motor Units & Muscle Mechanics

Type your answers, draw in the boxes or upload a photo of your hand drawing, then use Save as PDF to submit. Your work saves on this device as you go.

Paste attempts0

Your work autosaves on this device.

Open the full notes →

Part 1 of 4 · Recall

Fill in the blanks

Type the term that completes each statement, using the word bank. Pull it from memory first.

Word bank

RecruitmentEccentricAnaerobic glycolysisUnfused tetanusIsometricType I (slow oxidative)Direct ATPToo shortFused (complete) tetanusSmall motor unitMotor unitIsotonicTwitchSize principleType IIx/b (fast glycolytic)

  1. one motor neuron + all muscle fibers it innervates
  2. few fibers per neuron · fine control (extraocular ~3-5 fibers)
  3. small motor units fire first; larger added as needed
  4. activating more motor units to increase force
  5. red, lots of mitochondria, fatigue-resistant · endurance
  6. white, anaerobic, fast and powerful, fatigues quickly
  7. response to one action potential · latent, contraction, relaxation phases
  8. rapid stimulation; partial relaxation between twitches
  9. so rapid no relaxation; smooth sustained contraction
  10. thick filaments collide with Z discs; force drops
  11. muscle changes length, load constant (lifting a weight)
  12. muscle generates force without changing length (holding a plank)
  13. isotonic lengthening under load (lower the curl — this is what makes you sore)
  14. lasts ~2 seconds
  15. ~30-60 seconds; produces lactate

Define it: high-yield vocabulary

Write a clear definition in your own words for each term.

  1. Motor unit
  2. Size principle
  3. Recruitment
  4. Type I (slow oxidative) fiber
  5. Type IIx (fast glycolytic) fiber
  6. Twitch
  7. Tetanus
  8. Isotonic contraction
  9. Isometric contraction
  10. Concentric contraction
  11. Eccentric contraction
  12. Creatine phosphate

Part 2 of 4 · Anatomy lab

Draw and label

Box A. Two motor units, side by side

Directions

  1. On the LEFT, draw a small circle for a motor neuron cell body in the ventral horn of the spinal cord. Extend an axon downward.
  2. Branch the axon into 5 short terminals, each ending on a different muscle fiber. Draw 5 short ovals as the muscle fibers.
  3. Label this side Small motor unit (eye muscle).
  4. On the RIGHT, draw another cell body and axon. Branch it into many terminals (draw 12 to 20). Draw the same number of muscle fibers.
  5. Label this side Large motor unit (quadriceps).
  6. Add labels: Motor neuron cell body, Axon, Axon terminal, Neuromuscular junction, Muscle fiber.
Color Size Tool
Your uploaded drawing for Box A. Two motor units, side by side

Box B. Twitch summation (force vs time)

Directions

  1. Draw an x-axis (time) and a y-axis (force).
  2. On the same axes, sketch three force traces stacked vertically by frequency.
  3. Trace 1: single twitches at 1 Hz. Force rises and falls completely between each stimulus. Label Single twitches.
  4. Trace 2: stimulation at 10 Hz. The second twitch starts before the first finishes; force adds up. Label Wave summation.
  5. Trace 3: stimulation at 30 Hz. Twitches fuse into a smooth, sustained plateau. Label Complete tetanus.
  6. Below the graph write one sentence: why does higher frequency produce more force?
Color Size Tool
Your uploaded drawing for Box B. Twitch summation (force vs time)

Structures to label

Label each on your drawing.

  1. Motor neuron cell body
  2. Axon
  3. Axon terminal
  4. Neuromuscular junction
  5. Muscle fiber
  6. Small motor unit
  7. Large motor unit
  8. Single twitch
  9. Wave summation
  10. Complete tetanus

Part 3 of 4 · Physiology lab

Reason it through

A. Fiber type comparison table

Which fiber type would dominate the postural muscles of the back? Justify in one sentence.
A 100-meter sprinter and a marathon runner are tested. Whose calves would have a higher percentage of Type IIx fibers? Whose would have the most mitochondria? Justify each.

B. Synthesis

1. Eye muscles are innervated by very small motor units (only a handful of fibers each). Explain why this design works beautifully for precision tracking but would fail for lifting a heavy object.
2. Apply the size principle. A person picks up a coffee cup. Then the same person attempts a deadlift. Which motor units are recruited in each case, and in what order?
3. Train a sprinter on explosive jumping and a marathoner on long slow distance for six months. Predict which fiber type each adapts most strongly and the physiological mechanism behind the adaptation.

Submit

Save as PDF, then upload to Canvas.

The exported PDF stamps your name and paste-attempt count. Drawn-here or hand-drawn diagrams only; typed or AI-generated diagrams are not accepted.

Submit on Canvas →